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Models or Theories
• Allow scientists to communicate their ideas to others.
• A model should be able to explain previous observations and predict future outcomes.
HISTORY OF THE ATOMHISTORY OF THE ATOM
460 BC Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to
smaller and smaller particles which he
called
ATOMAATOMA
(greek for indivisible)
HISTORY OF THE ATOMHISTORY OF THE ATOM
1808 John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce
around with perfect elasticity and called
them
ATOMSATOMS
Dalton’s Atomic Theory
• All matter is made up of small particles called atoms.
• Atoms cannot be created, destroyed or divided into smaller particles.
• All atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements.
• Compounds are created when atoms of different elements link together in definite proportions.
Thomson’s Experiment: Cathode Rays
Passing an electric current makes a Passing an electric current makes a beam appear to move from the negative beam appear to move from the negative to the positive end. (By adding an electric to the positive end. (By adding an electric field, he was able to conclude that the field, he was able to conclude that the particles were negative.)particles were negative.)
HISTORY OF THE ATOMHISTORY OF THE ATOM
1898 Joseph John Thompson
found that atoms could sometimes eject a
far smaller negative particle which he
called an
ELECTRONELECTRON
HISTORY OF THE ATOMHISTORY OF THE ATOM
Thompson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere
surrounded by a soup of positive charge to balance the
electron's charge
1904
like plums surrounded by pudding.
PLUM PUDDING
MODEL
HISTORY OF THE ATOMHISTORY OF THE ATOM
1910 Ernest Rutherford
fired + particles (alpha particles) at a thin
sheet of gold foil, he expected the
particles to go straight though
HISTORY OF THE ATOMHISTORY OF THE ATOM
gold foil
helium nuclei
They found that while most of the positive particles
passed through the foil, a small number were deflected
and, to their surprise, some positive particles bounced
straight back.
helium nuclei
HISTORY OF THE ATOMHISTORY OF THE ATOM
Rutherford’s new evidence allowed him to propose a
more detailed model with a central nucleus.
He suggested that the there was a small, dense,
positive charge in a central nucleus. With this holding
the large, negative electron cloud in place by electrical
attraction.
However, this was not the end of the story.
HISTORY OF THE ATOMHISTORY OF THE ATOM
1913 Niels Bohr
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by
adding that the electrons were in
orbits. Rather like planets orbiting the
sun. With each orbit only able to
contain a set number of electrons.
Bohr’s Atom
electrons in orbits
nucleus
HELIUM ATOM
+N
N
+-
-
proton
electron
neutron
Shell
What do these particles consist of?
ATOMIC STRUCTUREATOMIC STRUCTURE
Particle Charge Mass Found
Proton +ve heavy nucleus
Neutron neutral heavy nucleus
Electron -ve light outside nucleus
ATOMIC STRUCTUREATOMIC STRUCTURE
the number of protons in an atom
the number of protons and neutrons in an atomHeHe
44
22 Atomic number
Atomic mass
number of electrons = number of protons
ATOMIC STRUCTUREATOMIC STRUCTURE
Electrons are arranged in Energy Levels
or Shells around the nucleus of an atom.
• first shell a maximum of 2 electrons
• second shell a maximum of 8
electrons
• third shell a maximum of 8
electrons
• fourth shell a maximum of 2
electrons
SUMMARYSUMMARY
1. The Atomic Number of an atom = number of
protons in the nucleus.
2. The Atomic Mass of an atom = number of
Protons + Neutrons in the nucleus.
3. The number of Protons = Number of Electrons.
4. Electrons orbit the nucleus in shells.
5. Each shell can only carry a set number of electrons.
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